This invention generally relates to providing a resinous coating on metal sheet and a metallic container wall. In addition, and more particularly, it relates to a metallic container wall having on its external surface an epoxy coating containing a lubricant. The coating substantially eliminates metallic slivers and metal fines caused by severing the end wall with a conventional plow-type can opener.
For many cans not having easy opening devices, it is necessary to open them by conventional mechanical can openers. Most can openers, whether manually or electrically operated, include a cutting edge for severing the container end wall and gears for engaging the seam of the container end to rotate the container with respect to the cutter. When opening a container, a can opener usually contacts a container end wall at several places in addition to the line of severance. One problem with conventional can openers is the likelihood of producing slivers and metal fines resulting from the severing of the end panel. Such slivers and metal fines are the direct result of the friction and abrasion on the metallic surface of the container by the cutting edge and other contact areas of the can opener. The small metal particles can fall into the container and be deleterious to its contents, such as food stuffs, during the cutting operation or during the removal of the end panel.
Slivering generally becomes more of a problem when container end panels are made of aluminum or its alloys. Generally, aluminum and its alloys exhibit a coefficient of surface friction relatively higher than other metals. Aluminum end panels tend to offer a greater sliding friction with the contacting point of a can opener than, for example, steel or tinplate container end panels. The higher frictional resistance tends to cause more slivers and metal fines than such non-aluminum end panels.
Additionally, container opening techniques can vary between individuals. It has been found that the force applied by individuals to can openers, especially to manually operated openers, affects the degree of slivering of container end panels, regardless of the metal of the panels. The amount of friction depends in part on the force applied.
Another variable affecting the slivering problem is the inherent tolerances between each can opener, even those made by the same manufacturer. As can openers vary, the number of contact places and the force of contact with a container end panel will vary resulting in varying degrees of slivering.
To solve the slivering problem, particularly of aluminum ends, a solution which overcomes or eliminates the problem variables is required. Ideally, opening an aluminum container end panel without slivers and metal fines should be independent of the can opener used and any individual techniques of opening.
Various approaches have been taken in the prior art to deal with problems relating to the opening of metal container ends using can openers. In general, the approaches involve relocating the severance of the end panel to an outer surface of the double seam joining the panel to the container body. U.S. Pat. No. 2,384,042, issued Sept. 4, 1945, discloses a closure being removed by cutting the seam on the lower side and outside of the container wall so that metal particles do not fall into the container when the closure is removed. U.S. Pat. No. 2,311,001, issued Feb. 16, 1943, also relates to severing outside the seam and uses a sealing compound within the folds of the container double seam. The raw edge of the container body is embedded in the sealing compound where the cutting occurs. A specifically designed can opener and container end seam construction are discussed in U.S. Pat. No. 3,139,211, issued June 30, 1964, for the purpose of avoiding penetration and thus contamination of the interior of the can by the can opener, the can cover or any operation associated with the opening procedure.
It has also been proposed that slivering of aluminum container ends can be substantially eliminated by, singly or in combination, modifying the profiles of end panels, changing alloy composition of end panels and modifying conventional can openers. End profiles have been changed to include recesses about the panel periphery where severance occurs. Also, score lines have been used at the line of severance to reduce metal thickness. Various aluminum alloys were tried and some were found to reduce the slivering problem of container ends made of those alloys. Modified conventional can openers with smaller controlled tolerances and a reduced angle of the cutting edge of the opener plow face have also minimized slivering. While these proposed approaches can be somewhat successful in minimizing slivering, they can be impractical and uneconomical for the can industry to implement. Other more practical solutions to the slivering problem are needed.
It is also known to coat can sheet on one or both surfaces with a thermoplastic material prior to blanking can ends from the sheet, as is shown in U.S. Pat. No. 2,086,165 issued July 6, 1937. A metallic container wall may also be provided with a laminate on its interior surface that withstands scoring without fracturing entirely therethrough, as is shown in U.S. Pat. No. 3,632,461 issued Jan. 4, 1972. As barrier layer of polyethylene may be secured to the container wall by an epoxy adhesive and an outer protective layer of polyethylene may be secured to the barrier layer by an adhesive of epoxy to protect the container wall from the contents of the container, and vice versa.
Applying a coating containing a lubricant to metal before working, such as by drawing and shaping, is also known in the art. Coating a metal with a lubricant consisting essentially of a solid high molecular weight polymer such as polyethylene, having a long carbon chain, is shown in U.S. Pat. No. 3,250,103 issued May l0, 1966. The polypropylene may be used alone or modified with a wax. A lubricant formed by a dispersion of cellulose ethers with one or more of other ingredients, such as polyethylene, is disclosed in British Pat. No. 1,004,836. U.S. Pat. No. 3,478,554, issued Nov. 18, 1969 and assigned to the common assignee of the present invention, discloses a method of drawing metal sheet having a resinous coating containing a lubricant. The resinous coating of that patent may be of the epoxy type containing a lubricant, such as polyethylene, which is from 2 to 6% by solid weight of the coating.
Even though it is known for metal containers to have thin exterior coatings to improve handling of the container and/or its aesthetic appearance, and to protect a container from its environment, there still exists a need in the prior art for substantially eliminating slivers and metal fines caused by the use of conventional can openers on metallic can ends such as those made of aluminum. The problem solution should be independent of alloy compositions, profiles of the end panels, individual can opening techniques and the can opener used. It is desirable that a container end wall be provided which is economically compatible with conventional can making, which has improved opening characteristics and for which no special or non-conventional can opener is needed.